1. Field of the Invention
The present invention generally relates to a servo unit. More particularly, the invention relates to an electromagnetic servo unit of the type in which input torque is electromagnetically servo-controlled to be output.
2. Description of Relevant Art
Most of conventional servo systems, exemplarily of those applied to power steering systems for vehicles, are constituted mainly by use of a hydraulic mechanism.
Such a conventional hydraulic servo system, as it is applied to a power steering system for vehicles, includes a constitution in which hydraulic oil from a hydraulic pump is sent under pressure into a servoactuator such as a hydraulic power cylinder or hydraulic servomotor for servo control of input torque from a steering wheel, to provide a controlled output for an operating element to be actuated, in accordance with operational condition of the steering wheel.
In such a hydraulic servo system, however, there are involved problems such that a hydraulic pump is normally put in service, whether or not an effective input is given from a steering wheel, thus uselessly consuming power, and that not a few component parts are employed such as an actuator, hydraulic pump, control valve, and oil reservoir, occupying a relatively large space as a whole, raising difficulty such as when trying to attain a compact, lightweight design and cost economy.
To overcome such problems of a hydraulic servo system, there have been already proposed a number of electromagnetic servo systems; exemplarily in U.S. Pat. Publication No. 2,754,465 in which an electromagnetic servo system is applied to a power steering system for vehicles. According to this U.S. Patent, the electromagnetic servo system includes a direct-current motor coaxially arranged on a steering shaft interposed between a steering wheel as an input member of the power steering system and a gear as an output member thereof, and a torque sensor of a well-known strain gauge type attached to the steering shaft, the torque sensor detecting torque as applied from the steering wheel to the steering shaft, to thereby control the direct-current motor.
Though having effectively solved conventional problems attendant the hydraulic servo system to certain extent, the electromagnetic servo system according to the U.S. Patent, in which the strain gauge type torque sensor is not provided with sufficient consideration for protection, stands susceptive to disturbances such as by dust, moisture, and external forces.
Moreover, in the electromagnetic servo system, power assistance by torque is directly made to the steering shaft from the direct-current motor which, thus, has to be of a high-output type and hence be of a large size, so that the entire system is sized relatively large.
Further, in the electromagnetic servo system, the strain gauge type torque sensor, as well as a limit switch for detecting the direction of stress developed in the steering shaft, is disposed at a separate point on the steering shaft with respect to the direct-current motor, and besides, at a place separated from the steering shaft, an amplifier with a power unit is provided, receiving signals from the torque sensor and the limit switch, for controlling output torque of the direct-current motor. As a result, the entire system is complicated.
On the other hand, there has been proposed another electromagnetic servo system in Japanese Patent Lay-Open Print No. 58-141963 laid open on Aug. 23, 1983. The electromagnetic servo system according to this Japanese Lay-Open Print, which is applied to a power steering system for vehicles, includes an electromagnetic motor coaxially arranged on a pinion shaft of the power steering system of a rack and pinion type, and a planetary gearing coaxially arranged on the pinion shaft, through which gearing an output of the motor is reduced of rotation speed before being applied to the pinion shaft, so that torque of the output applied to the pinion shaft is magnified the more for the speed reduction through the planetary gearing.
In the electromagnetic servo system according to the Japanese Lay-Open Print, the provision of the planetary gearing as a reduction gearing effectively eliminates the need of using a motor so high of the output.
However, also in the electromagnetic servo system according to the Japanese Lay-Open Print, there are separately arranged with respect to the electromagnetic motor a torque detector for detecting torque applied to the pinion shaft as an input side member and a control mechanism receiving a detection signal from the detector to thereby control output torque of the motor. As a result, like the case of the aforementioned U.S. Patent, the servo system is susceptive to disturbances and, besides, complicated.
Further, in the servo system according to the Japanese Lay-Open Print, the electromagnetic motor is controlled so as to always have torque output in proportion to the steering torque as applied to the pinion shaft from a steering wheel, thus being inherently bound to unnecessary consumption of power.
The present invention has been achieved to effectively solve such problems of a conventional electromagnetic servo system, in a further improved form.